Method of coating wire-wound electrical resistors



Mardi 4, 1947. B. c. BRICKER METHOD OF COATING WIRE-WOUND ELECTRICAL RESISTORS Filed Aug. 22. 1944 i N V EN TOR.

l' Y ATTRNEY Pstented Mar. 4,1 .1941

METHGD F COATING WIRE-WOUND ELECTRICAL RESISTORS Burton C. Bricker, Rahway, N. J., asslg'nor to E. I. du Pont de Nemours & Company, Wilmington, Del., a corporation of Delaware Application August 22, 1944, Serial No. 550,635

3 Claims.

Awhich will have a substantially smooth uniform coating of vitreous enamel and whichv will resist' sudden changes in temperature without fracturing. Particular dificulty has been met by axial shifting of the coils of the spirally wound resist- .ance wire during the firing of the enamel coating thereon. 'I'he wire coils separate in bunches along the core as a result of high surface tension of the fused enamel.

As the enamel,fuses during the firing thereof,

The resistor core is generally composed of a ceramic material, for example, steatite porcelain, magnesium silicate or magnesium aluminum silicate. A preferred core is one composed of a ceramic material having a linear expansion cothe enamel coating sometimes splits and the surface tension on the fused semi-liquid enamel causes the coating to draw away from the split' thereby resulting in a widening of the split and often leaving a considerable' gap in the coating. The wire coils,v are also pulled away from each other at the said. gap and thereby being drawn into bunches `along the core. For obvious reasons this is very objectionable. y

The above-described objectionable phenomenon has been remedied to some extent by rst coating the wire-Wound core with a thin liquid slip of kaolin clay or ball clay and drying the. same before application of the vitreous enamel slip and the firing thereof.

It is an object of this invention ,to provide an improvedclay slip which can be applied to a wirewound resistor as an under-coat for a vitreous enamel to prevent the splitting of the enamel coating. and the bunching o-f wires, during the firing of the enamel.

It is another object of this invention to provide an improved method for the production of wire-wound enamelled electrical resistors which will be free from split coatings and wire-bunching.

Other objects of the invention will appear here.

. inafter.

The above objects may be accomplished, in gen- The resistor core is wound in a conventional manner with an electrical resistance wire, for example, a nickel-chromium alloy or nickel-copper alloy Wire, and the wire securely held in place by means of metal fastening tabs or connecting elements to which the ends of the wire are fastened to form an electricity conducting unit from one tab, through the wire to the other tab.

The above-described wire-wound core is then provided with a thin coating of a slip composed of clay (kaolin clay or ball clay) and bentonite suspended in Water. Of thev total solids in the slip 98% to 50% is clay and 2% to 50% is bentonite. The slip should be made sufficiently uid to ilow between the Wire coils and to drain from the spaces between wire coils to leave a.' substantially uniform coating on the exposed surfaces of the core and wire. It will be readily apparent that if the slip is thick, it vwill not flow between wire coils but will bridge the space between coils, and as a result a coating of uniform thickness on the wires and the core, will not be obtained.

Thevthinness or fluidity of the slip will depend in large measure upon the size of the Wire, the spacing between wires, and the method of application. Sometimes the wire-woundA core is heated before dipping in the clay-bentonite slip. Under such circumstances, the slip must be thinner than if the core is cold when dipped. Some,- times a wire-wound core is wetted with water before dipping in the slip. Under such conditions, the slip must be somewhat thicker than if 'it is dipped while dry. ,In general, the clay-bentonite slip should contain between 3 and 30 parts water for 1 part solids, and preferably between 5 and 10 parts water for 1 part solids.

The slip is preferably prepared by mixing the clay and bentonite with water in the proportion of 1 part solids to 2 or 3 parts water and ballmilling'the mixture, for a period of between l to 3 hours. The ball-milled product is then diluted with Water to the desired consistency.

After application of the clay-bentonite slip,

preferably the latter, the wire-wound core is vpermitted to drain, for example, by suspending ever, forced air drafts, or even mild baking, can be employed, if desired, to dry the same.

The wire-wound core containing the dry coating of the clay-bentonite mixture is then coated with a vitreous enamel slip in a known manner. Any vitreable enamel may be now used for coating of the lresistor unit without material danger j of splitting of the enamel, or wire-punching, during firing of the enamel. The enamel slip is then dried and red in the conventional manner. Al-

though a vitreous enamel of any desired composition can be applied and fired with good results on the wire-wound core containing the claybentonite coating in accordance with this invention, outstanding results are obtained with enamelsl having a low coeicient of expansion. Enamels having a low coefficient of expansion are usually particularly subject to enamel splitting and wire-bunching.

The clay-bentonite under-coating of this invention has particular utility in the production of wire-wound resistors in accordance with the invention disclosed in the copending application of Alden J. Deyrup, Serial No. 551,041, filed August 24, 1944. The enamel coating disclosed in the above-said application contains lead oxide, zinc oxide, boric o-xide, alumina and silica in the following proportions by weight.

Per cent Lead oxide 28 to 55 Zinc oxide to 20 Boric oxide e to 30 Alumina 4 to 10 Silica 11 to 32 and also, if desired, small proportions lof other compounds, for example, cobalt oxide, nickel oxide, chromium oxide and zirconium oxide.

These enamel compositions have a very high resistance to fracturing when subjected to sudden changes of temperature. However, these enamels are especially subject to enamel splitting and wire-bunching during firing. By rst undercoating the wire-wound core with a clay-bentonite slip, in accordance with the present invention, a particularly desirable-enamelled electrical resistor can be produced by the method of said application. By the use of a clay-bentonite undercoating enamelled resistors can be produced with the enamelsdisclosed in said application With. out waste or spoilage due to wire-bunching or enamel-splitting. The specific combination of a clay or clay-bentonite under-coating with the enamel coatings of the above-mentioned Deyrup application which leads to new and unobvious yresults is the subject matter of my copending application Serial No. 553,367, filed September 9, 1944.

'Ihe clay-bentonite under-coating of the present invention permits the formation of more uniform and more adherent coatings on Wire-Wound cores for the prevention of enamel-splitting and bentonite under-coating and a vitreous enamel slip over-coating,

Fig. 2-is asimilar fragmentary sectional view of the finished 'electrical resistor after firing of the structure shown in Fig. 1.

Referring to Fig. 1 of the drawings, referenceV numeral 6 designates a tubular ceramic core, upon which is spirally wound a resistance wire 8. The

end of the wire 8 is welded, brazed, or otherwise fastened at I I to metallic tab I0 which is clamped firmly about the core 6. A clay-bentonite composition of the type described above, designated by numeral I2, is shown in a substantially uniform layer about the core Ii, the tab I0 and the wire 8. Although, in the stageof production' shown in this drawing, the clay-bentonite is dry and contains an overcoating I4 of enamel slip, it will be readily apparent that the clay-bentonite, when applied as a slip (aqueous suspension), was sufciently thin and uid to ow in between wire coils and to drain from the spaces between the coils to leave a more or less uniform coating.

' In Fig. 2 of the drawings, in which'like reference characters designate likeparts, the enamel Throughout the specification, reference to -parts, percentages and proportions refers to parts, percentages and proportions -by weight,

unless otherwise specified.

Since it is obvious that many changes and modifcations can be made in the above-described details without departing from the nature and spiritoi' the invention, it is to be understood that the invention is not limited by said details except as A set forth in the appended claims.

I claim:

1. The method of making wire--woundv electrical resistors which comprises applying to a core wound with resistance wire a coating of a slip comprising an aqueous suspension of a clay taken from the group consisting of kaolin clay and ball clay, and bentonite in a proportion of 98% to 50% clay and 2% to 50% bentonite, the slip being.

sufliciently fluid to flow between the wire windings and to drain from the spaces between wind ings lto leave al substantially uniform coating on the surfaces contacted, drying said coating, ap-

plying a coating of a vitreous enamel slip over said dry clay-bentonite coating, drying said enamel slip, and ring said enamel.

2. The method of making wire-wound electrical resistors which comprises applying to a, core wound with resistance wire a coating of a slip comprising an aqueous suspension of a clay taken from the group consisting of kaolin clay and ball clay, and bentonite in a proportion of 98% to 50% clay and 2% to 50% bentonite, said slip contain.- ing 3 to 30 parts by weight of water for each part solids and being sufficiently fluid to iiow between windings to leave a substantially uniform coating on the surfaces lcontacted, drying said coating, applying a coating of vitreous enamel slip over said dry clay-bentonite coating, drying said enamel slip, and firing said enamel.'

3. The -method of making wire-wound electrical resistors which comprises applying to a core Wound with resistance wire a coating of a slip comprising an aqueous suspension of a clay. taken from the group consisting of kaolin clay and ball UNITED STATES PATENTS clay, and bentonite in a proportion of 98% to 50% .Numb clay and 2% to 50% bentonite, said slip containing 78785.47 Leolrlxe Aprnfelgof, 5 to 10 parts by weight of water for each part 1 365331 Mccunoci Jan' 11 1921 solids and being sulliciently fluid to ow between 5 1530928 Braun "Mar' 24 1925 windings to leave a substantially uniform coating 1787749 Heyrot Ja'n 6 1931 on the surfaces contacted, drying said coating, ap- 1989736 Boyles Feb 5 1935 plying a, coating of vitreous enamel slip over said 2:213969 Ruben :::::::""ept '10 1940 dry clay-bentonite coating, drying said enamel 2,385,702 Hediger et al. Sept'- 25, 1945 Slip and ming Said enameL 1 2,134,752 Ehlers N0v 1I 1933 1,948,382 Johnson Feb. 20, 1934 BURTON C' BRICKER' 2,118,798 Saxe May 24, 1933 REFERENCES CITED v OTHER. REFERENCES The following references are of record in the u Bentonite Information Bulletin No. 98, May 18, l

le of this patent: 1939. (See Division 38.) 

